In recent years, the spread of various kinds of electric automobiles has been expected as the solution to the environment and energy issues. The development of the lithium ion secondary battery as an on-vehicle power source such as a power source for driving a motor, which is the key for the practical application of the electric automobiles, has been conducted extensively. For widely spreading the battery as the on-vehicle battery, however, it is necessary to increase the performance of the battery and decrease the price of the battery. Moreover, the mileage per charge of the electric automobile needs to be as long as that of the automobile powered by a gasoline engine. Thus, the battery with higher energy has been anticipated.
For allowing the battery to have higher energy density and higher discharge capacity, it is necessary to increase the quantity of electricity to be stored in a positive electrode and a negative electrode per unit mass. As a positive electrode material (active material for a positive electrode) that can meet the above requirements, a Ni—Co—Mn ternary composite oxide and a so-called solid solution based material having a crystal structure belonging to a space group R-3m and containing lithium in a transition metal site have been examined. In the Ni—Co—Mn ternary composite oxide and the solid solution based material, however, since the irreversible capacity at the initial charge is high, the initial charge-discharge efficiency of the positive electrode active material is low. Therefore, in the design of the battery, the opposite negative electrode needs to be used excessively, resulting in a problem that the battery capacity deteriorates, for example. In view of this, Patent Literature 1 has suggested that the initial charge-discharge efficiency is improved by the inclusion of the lithium-contained metal oxide containing nickel and manganese, and LiFePO4.